FLASH memory devices are utilized for a variety of applications. FIGS. 1A-1F illustrate a first conventional process for forming a portion of a FLASH memory device. First, a substrate 100 is formed, as illustrated in FIG. 1A. Then the substrate 100 is lightly doped, typically with Boron, to form a p-type substrate, as illustrated in FIG. 1B. A layer of field oxide 120 is grown on the surface of the substrate 100, as illustrated in FIG. 1C. The field oxide 120 is further processed using known methods and results in the oxide layer 140, as illustrated in FIG. 1D. The gate 130 is then defined also using known methods, as illustrated in FIG. 1D.
Under this first conventional method, as illustrated in FIG. 1E, the source junction structure is formed by a high-dose implantation of arsenic into the substrate 100. The resulting source junction structure is shown in FIG. 1F.
A common problem in the above structure is the existence of band-to-band tunneling current. It is well known that steep doping gradient as found in a single-implant, arsenic-only source profile (see FIG. 2) of a FLASH memory device can cause high and deleterious levels of band-to-band tunneling current during the erase operation and that the introduction of some phosphorus at the junction and near the surface of the substrate under the gate can alleviate this problem. Dotted line 150 on FIG. 1F indicates the line along which the doping profile of FIG. 2 is represented. The portion of the source doping profile under the gate 130 provides for efficient erasing of the FLASH memory device.
FIGS. 3A-3D illustrate a second conventional process which introduces phosphorus into the substrate. Typically, the phosphorus is introduced after the doping of the substrate 300, the growing of the oxide layer 320, and the definition of the gate 330, as described with respect to the first conventional process, as described in FIG. 1A-1F. As illustrated in FIG. 3A, the phosphorus is usually introduced by implanting it into the substrate 300. A high-dose of arsenic is then implanted, as illustrated by FIG. 3B. Then the phosphorus and arsenic laterally diffuse to their location at the junction area of the substrate 300, as illustrated by dotted circle 340 in FIG. 3C. This subsequent diffusion of the high-dose arsenic results in the final source structure.
The resulting arsenic-phosphorus source junction structure is illustrated in FIG. 3D. Since the phosphorus diffuses further into the substrate than the arsenic and because of the general characteristics of phosphorus diffusion profiles, it has a more graded lateral concentration profile, i.e., the slope of the profile is less steep. This combination profile, shown in FIG. 4, does reduce band-to-band tunneling current, but it creates another problem, that of the so-called short channel effect. The short-channel effect is due to the phosphorus creeping into other regions of the substrate, due to the depth of the initial implant and the diffusion of the phosphorus in all directions, as illustrated by arrows 360 in FIGS. 3C-3D. Dotted line 370 on FIG. 3D indicates the line along which the doping profile of FIG. 4 is represented. Referring back to FIG. 4, line 400 represents the substrate/channel doping level. Where the line 400 intersects the phosphorus lateral concentration profile marks the doping concentration level of the source junction. The phosphorus represented by its lateral concentration profile above line 400 reduces the band-to-band tunneling current. However, the phosphorus has a "tail," represented by its concentration profile below the line 400. The tail of the phosphorus, also represented by arrows 360 of FIG. 3C-3D, has crept deeply into the substrate 300 beyond the source junction and into the doped channel beyond the source junction, thereby increasing deleterious short-channel effects.
There thus exists a need for a new method for forming a source junction structure in semiconductor devices which reduces band-to-band tunneling while also reducing short-channel effects. The method should be easily implemented utilizing existing technologies while also being cost effective. The present invention addresses such a need.